MOP questions (was Re: [Openmcl-devel] byte-strings)

Ben ben at medianstrip.net
Tue Aug 31 05:50:35 UTC 2004


thanks for the help.

now some MOP questions, relating to adding additional keywords to
slots:

1) it appears that direct-slot-definition-class is receiving initargs
sometimes inside of lists, e.g. like

(:name slot3 :allocation :class :transient (nil))

(transient is a slot keyword which we've added to our specialized
slot-definition.)

why is this? is it safe to assume this will always be the case?

2) i am having trouble using additional slot keywords, because
effective-slot-definition-class isn't getting the keywords.  looking
at the sources compute-effective-slot-definition, it is constructing
an initarg plist that doesn't include my :transient key.  this is is
contrary to my reading of the MOP "spec."

any ideas?

thanks in advance, B

On Fri, 20 Aug 2004, Gary Byers wrote:

>
>
> On Fri, 20 Aug 2004, Ben wrote:
>
>> hi,
>>
>> i'm writing an application which wants to treat strings as byte arrays
>> for the purposes of FFI.  more specifically, i'd like to be able to
>> quickly copy the contents of strings in and out of foreign character
>> arrays.  i don't really care about what the character representation
>> is, i'm happy treating everyting as bits.
>>
>> under CMUCL and Franz i can directly pass strings to memcpy, which
>> does the trick.  this works even with 16-bit strings in Franz,
>> provided i pass in the correct byte-length.  under Lispworks there is
>> a function "fli:replace-foreign-array" which i'm hoping will do the
>> trick for me.
>>
>> of course there is nothing stopping me from doing (using UFFI)
>>
>> (defun copy-buf (str buf len src-offset 0 buf-offset 0))
>>    (declare (optimize (speed 3) (safety 0))
>>             (type string str)
>>             (type array-char buf)
>>             (type fixnum len src-offset buf-offset)
>>             (dynamic-extent str buf len))
>>    (typecase str
>>      (simple-string
>>       (loop for i fixnum from 0 below len
>>             do
>>             (setf (deref-array buf '(:array :char) (+ i buf-offset))
>>                   (char-code (schar str (+ i src-offset))))))
>>      (string
>>       (loop for i fixnum from 0 below len
>>             do
>>             (setf (deref-array buf '(:array :char) (+ i buf-offset))
>>                   (char-code (char str (+ i src-offset))))))))
>>
>> but i find memcpy is much, much faster.  is there any way to get at
>> the underlying bits of a string?  e.g. is "with-cstr" using memcpy,
>> and can i pass it a buffer to stuff the string into?
>>
>
> WITH-CSTR uses something called CCL::%COPY-IVECTOR-TO-PTR, which takes
> 5 arguments:
>
> (ccl::%COPY-IVECTOR-TO-PTR     source			; an "ivector"
>                               source-byte-offset	; not bounds-checked
>                               dest			; a MACPTR
>                               dest-byte-offset		; no checking here ..
>                               nbytes)			; ... or here either
>
> An "IVECTOR" is basically any (SIMPLE-ARRAY <type> (*)), where
> <type> is something for which (UPGRADED-ARRAY-ELEMENT-TYPE <type>) is
> other than T.  (E.g., a SIMPLE-STRING, a bit vector, a vector of
> signed/unsigned bytes, a vector of some float type, etc.)  There are
> other, implementation-level things that are also ivectors.
>
> Note that a non-SIMPLE-STRING is -not- an ivector, but it's displaced
> to one (see below.)
>
> CCL::%COPY-IVECTOR-TO-PTR's written in assembler; it could be faster
> in some cases (using DOUBLE-FLOATs or AltiVec to move larger chunks
> of congruently-aligned stuff around), but in practice it's pretty fast.
>
> To copy the bytes in a SIMPLE-STRING to foreign memory:
>
> (ccl::%copy-ivector-to-ptr "ABC" 0 p 0 3)
>
> If a string isn't a SIMPLE-STRING, it's (perhaps transitively) displaced
> to one.  CCL::ARRAY-DATA-AND-OFFSET returns two values:
>
> (0) the underlying (SIMPLE-ARRAY * (*))
> (1) the cumulative displacement
>
> Calling CCL::ARRAY-DATA-AND-OFFSET on something that -is- a
> (SIMPLE-ARRAY * (*)) returns that argument and 0.
>
>> thanks in advance, B
>
> If you look at WITH-CSTR (I hadn't done so in a long time ...), you'll
> see a lot of code that's a little too general wrapped around a call to
> CCL::%COPY-IVECTOR-TO-PTR.  (Hmmm ... if you look at it as long as I
> just did, you'll also see that CCL::DEREFERENCE-BASE-STRING should
> just be returning the length of its argument as a third value, not
> (+ offset length) as it's been doing.)
>



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